The present invention relates generally to methods and compositions for melting snow and/or ice covered surfaces. In particular, this invention relates to a corrosion inhibited deicing composition and the methods for its use in melting snow and ice from roadway surfaces.
As a part of some state and local highway departments' "dry pavement" policies designed to minimize ice and snow related traffic accidents, melting agents for removing hazardous ice and snow are routinely applied to roadway surfaces. For some time, sodium chloride has been the most widely used melting agent. However, sodium chloride is strongly corrosive of many structural metals. Consequently, the current use of sodium chloride to melt snow and ice on roadways has resulted in serious corrosion to the metal parts of highway structures and to the vehicles that travel the treated surfaces. The worst damage from corrosion occurs on bridges. Since bridges are one of the most expensive structures in a highway system, considerable effort and expense are put forth annually to preserve and repair them.
Magnesium chloride is also known to effectively melt ice and snow and because it melts ice at a much lower temperature than sodium chloride, it is often a preferred melting agent. Magnesium chloride is especially preferred in areas where seasonal temperatures often fall well below the freezing point of sodium chloride solutions, rendering the sodium salt useless as a melting agent until the ambient temperature increases sufficiently. Many highway departments, therefore, use solutions of magnesium chloride to melt ice. Even though magnesium chloride is a better low temperature melting agent than sodium chloride, magnesium chloride is similarly corrosive to highway structures and vehicles.
In addition, magnesium chloride has been used as an additive in sodium chloride based melting agents. U.S. Pat. No. 3,505,234, issued to Pinckernelle in 1970, teaches that magnesium chloride (or calcium chloride) and an alkali salt of an iron cyanide, when added to a major portion of sodium chloride, prevent caking and freezing of the resulting deicing composition. The magnesium chloride may be in an aqueous solution (brine) which is added to the rock salt sodium chloride or it may be added dry.
Similarly, U.S. Pat. No. 3,772,202 issued to Neitzel in 1973 describes that magnesium chloride-containing brine solutions entrained in a solid sodium chloride phase results in a road salt composition which remains moist and spreadable at very low temperatures. The road salt composition is saturated in sodium chloride, the major component thereof. Ferrocyanides are an optional anti-caking ingredient. U.S. Pat. No. 3,833,504, also to Neitzel and issued in 1974, discloses a method for making these entrained brine compositions.
Efforts have been directed to minimizing the corrosive effects of road salt compositions. Such efforts mainly include the addition of corrosion inhibiting agents which apparently act to neutralize the tendency of the salt to corrode. It is known to use divalent metal ion salts and suitable counterions including chlorides in addition to polyphosphate salts to inhibit the corrosivity of sodium chloride. For example U.S. Pat. No. 4,803,007 issued to Garber in 1989 discloses an inhibited deicing composition which includes a major amount of sodium chloride and also a corrosion inhibitor. The corrosion inhibitor is preferably either a zinc halide, acetate, nitrate, oxide or sulfate or a divalent magnesium source. It is interesting to note that the divalent magnesium source disclosed in the patent may be corrosive magnesium chloride salt. In addition to the zinc and magnesium source, the inhibitor includes a polyphosphate salt which is tripolyphosphate, hexametaphosphate or a mixture thereof.
In combination with other materials, polyphosphates have proven useful in preventing oxidative corrosion in recirculating cooling water systems which also contain chlorine as an antibacterial agent. For example, U.S. Pat. No. 4,108,790 to Foroulis issued in 1978 describes the use of polyphosphates, including alkali metal polyphosphates such as sodium hexametaphosphate. Foroulis teaches that the polyphosphate, to be effective, must be used along with a gluconate or else rapid conversion to insoluble orthophosphates occurs which adds to the formation of undesirable scale in the water system. The combination of gluconate and polyphosphate is taught to produce a synergistic effect which is not achieved by using either ingredient alone.
Spent sulfite liquor has been used to reduce the corrosivity of alkali metal or alkaline earth metal chloride salts, including sodium and magnesium chloride salts. U.S. Pat. No. 4,668,416 issued to Neal in 1987 describes a process for making a deicing or dust control agent with reduced corrosivity which includes a spent sulfite liquor and a chloride salt in a proportion so that the lignosulfonates are present from about 1:25 to 15:1 of the metal chloride.
In contrast, the applicant has surprisingly discovered that polyphosphate salts may be used alone to effectively inhibit corrosion caused by magnesium chloride. The applicant is unaware of any use of polyphosphate salts to prevent corrosion caused by magnesium chloride. The inhibition is enhanced when zinc salts are added. Moreover, the applicant has discovered that if excess chloride ion (i.e. above that present as a counterion to magnesium) is kept to a minimum, the inhibitor is surprisingly more effective. The present invention thus limits the corrosiveness of ice and snow melting magnesium chloride salt, extending the life of highway structures and reducing maintenance thereon.